1. Field of the Invention
The present invention relates to a gas flow stand.
2. Statement of the Problem
Gas metering is used in many applications for measuring and/or controlling a volume of gas. Many manufacturing and production operations include gas metering, wherein a gas or gases are metered into a manufacturing process or metered out of a production process. A mass flow meter can be used to measure and/or meter out gases in precise quantities. Accuracy in the mass flow meter is therefore very important.
Manufacturers of mass flow meters typically calibrate their meters before the meters are shipped to customers. The calibration can include many types of calibration processes and apparatus. One common calibration approach is to compare a gas flow measurement produced by a meter under test against a known, highly calibrated mass flow measurement of the same gas flow stream. As a result, the meter under test can be compared to a known measurement standard for purposes of calibration. One such type of calibration apparatus is a gas flow stand.
FIG. 1 shows a prior art gas flow stand. The prior art gas flow stand can include a gas source, an inlet valve, a meter under test, and an outlet valve. The prior art gas source typically comprises a pressurized tank or bottle that is connected to the meter under test. The gas source container in the prior art is typically a single large gas container. In the prior art, the gas source is weighed to obtain an initial weight value, the gas source is then connected to the meter under test and the test is conducted. Subsequently, the gas source is disconnected from the meter under test and the gas source is weighed again to obtain a final weight value. This weight/mass difference is then used to calibrate the mass flow measurement through the meter under test. The calibrated meter can then be delivered to a customer.
The prior art approach has drawbacks. One drawback is that the prior art approach weighs only the gas source and not the entire gas flow stand. The prior art approach weighs the gas source when it is not connected to the gas flow stand. The prior art approach does not accurately or completely take into account the gas left in the gas flow stand. For example, a measurable quantity of gas can be retained in the gas flow stand between the inlet valve and the outlet valve. The prior art attempts to compensate for the gas in the gas flow stand by approximation. The prior art uses a known volume of the gas flow stand apparatus and converts the volume to a mass using a measured temperature and pressure of the gas. However, this involves volume, temperature, and pressure measurements, and therefore the prior art approach introduces additional error and additional uncertainty. The prior art does not directly measure the mass of gas held in the gas flow stand components.
Another drawback is that the prior art approach does not directly measure or accurately take into account the gas that is released to the atmosphere when the gas source is disconnected. The prior art assumes that all of the gas in the weight difference flows through the meter under test. By measuring only the gas source, and neglecting the gas in the gas flow stand components and released gas, the prior art does not achieve a satisfactory level of accuracy.